The present invention relates to adding circuits for adding electrical currents and in particular to current adding circuits which may be monolithically integrated and may therefore be used in integrated circuits to generate measurement signals relating to several different current flows.
These adding circuits output a current which is equal to the sum of the currents supplied as an input thereto, or output a current which is proportional to this sum, irrespective of the polarity of the currents being input thereto.
In the case of monolithically integrated current adding circuits it is generally preferably to attenuate all of the currents which are being acted upon in order to reduce the total supply current absorption which these adding circuits require for the processing of the currents being input thereto. The simplest current adding circuit used at present in integrated circuits to form the sum of the currents of opposite sign comprises, as shown in FIG. 1, a first, a second and a third current mirror circuit M1, M2, and M3 each having an input port and an output port in which there are inserted resistors designed to calculate a constant factor of proportionality between the currents flowing in these ports. In the drawings, the current mirror circuits, which may be constructed using those techniques known to persons skilled in the art, are shown by rectangular blocks in which the input and output ports with the resistors inserted therein are also shown symbolically.
The input ports are shown by small circles.
Each resistor is shown by a number and its value, expressed by means of a constant value R and a coefficient K of predetermined value, is shown adjacent to it. The input port of the first current mirror circuit M1, in which there is inserted a first resistor 1 of value R, forms a first input terminal of the adding circuit. The output port of this circuit M in which there is inserted a second resistor 2 of value KR is connected to the input port of the second current mirror circuit M2 in which there is inserted a third resistance 3 of value KR.
The input port of the third current mirror circuit M3, in which there is inserted a fourth resistance 4 of value R forms a second input terminal of the adding circuit to which a current of opposite polarity to the current which is supplied to the first input may be supplied in accordance with the usual prior art techniques which are known to persons skilled in the art.
The output ports of the circuits M2 and M3 in which there are respectively inserted a fifth resistance 5 of value KR and a sixth resistance 6 also of value KR are connected together in a circuit node S which forms an output terminal of the adding circuit.
The adding circuit shown in FIG. 1 operates when currents having, as mentioned above, opposite polarity, for example--a current IA entering the first input terminal and a current IB being discharged from the second input terminal, are supplied to the two input terminals.
Since the ratio of resistance values between the resistors 2 and 1 is equal to K, this produces at the output port of the circuit M1 an output current IA/K which is K times lower than that of the current IA.
The current IA/K which is discharged from the current mirror circuit M2, to whose input port it is supplied, produces at the output port of this circuit an identical output current IA/K since the resistance values of the resistors 3 and 4 are the same.
The current IB also produces at the output port of the current mirror circuit M3 an output current IB/K which is K times lower than that of the current IB, since the ratio between the resistance values of the resistors 6 and 4 is equal to K.
The two currents IA/K and IB/K flowing in the circuit node S are therefore added therein, so that an output current IS=[IA+IB]/K is available at the output terminal of the adding circuit, this current being equal to the sum of th currents IA and IB multiplied by the coeffecient 1/K, and is in particular equal to the sum when K=1.
An mentioned above, it is preferable to attenuate all the currents which are being acted upon and therefore a value K which is greater that 1 is usually selected.